Cut the potatoes into quarters and place in a large saucepan with the garlic cloves. Fill the pot with water and season generously with salt. Taste the water, it should taste salty. Bring the pot to a boil over medium heat and cook the potatoes until they are fork tender. (A fork, not a knife should slide in and out easily.)
Preheat the oven to 350 degrees F.
When the potatoes are tender, drain the water from them and return the potatoes and garlic to the pan. Add the milk and sour cream. Using a potato masher or big spoon, gently mash the potatoes. Stir in the Cheddar, then taste and season with salt, if needed. Transfer the mixture to a baking dish. Combine the panko, grated Parmesan and the cayenne in a small bowl. Sprinkle it over the top of the potato mixture and bake until hot all the way through and is crispy and brown on top, about 20 minutes.
Remove from the oven, garnish with chopped chives and serve.